In processing of a melt-processable polymer, the polymer needs to be extruded at a high speed for improvement in the productivity and reduction in the cost. However, every melt-processable polymer composition has a critical shear rate, and an extrusion speed higher than this rate causes a phenomenon called melt fracture in which the surface becomes rough, which fails to produce a favorable molded article.
One method of eliminating the problem to achieve a higher extrusion speed and enhance the extrusion properties without causing melt fracture is a molding method performed at a higher molding temperature. However, the high-temperature molding involves pyrolysis of the melt-processable polymer which causes problems such as a decrease in the mechanical properties of the resulting molded article and discoloring of the molded article. Also, the high-temperature molding involves a decrease in the melt viscosity of the melt-processable polymer, which unfortunately leads to sagging or deformation before the molded article is solidified by cooling, decreasing the dimensional accuracy of the molded article.
In view of these problems, Patent Literature 1 discloses a process of producing an extrudable composition, including mixing together simultaneously i) 0.001 to 10 weight percent, based on total weight of the extrudable composition, of a first fluoroelastomer having a first Mooney viscosity ML (1+10) at 121° C. measured in accordance with ASTM D-1646, ii) 0.001 to 10 weight percent, based on total weight of the extrudable composition, of a second fluoroelastomer having a second Mooney ML (1+10) at 121° C. in accordance with ASTM D-1646, and iii) a non-fluorinated melt processable polymer, wherein the difference between said first and second Mooney viscosities is at least 15.
Patent Literature 2 discloses a method including the steps of forming a melt processable polymer composition comprising a melt-processable thermoplastic host polymer and an effective amount of a processing additive composition containing a specific multimodal fluoropolymer; mixing the processing additive composition and the host polymer for a time sufficient to blend them together, and melt processing the polymer composition.
Patent Literature 3 discloses, as a technology utilizing a fluoropolymer as a processing aid, an extrudable composition containing a thermoplastic hydrocarbon polymer, a poly(oxyalkylene) polymer, and a fluorocarbon polymer. Patent Literature 4 discloses an extrudable composition comprising: a resin blend comprising a metallocene catalyzed linear low density polyethylene resin and a low density polyethylene resin; a fluoroelastomer having a Mooney viscosity ML (1+10) at 121° C. between 30 and 60; and an interfacial agent. Patent Literature 5 discloses a processing aid comprising a fluoropolymer having an acid value of not lower than 0.5 KOHmg/g.
These disclosed technologies, however, cannot achieve the effect of preventing occurrence of melt fracture under high shear rate conditions where the shear rate is higher than 800 sec−1.
In order to prevent occurrence of melt fracture under high shear rate conditions where the shear rate is higher than 800 sec−1, Patent Literature 6 discloses a processing aid composition for difficultly melt-processable polymers, consisting essentially of: 2-95 parts by weight of a fluorocarbon copolymer which at the melt-processing temperature of the difficultly melt-processable polymer is either in a melted form if crystalline or is above its glass transition temperature if amorphous; and 98-5 parts by weight of a tetrafluoroethylene homopolymer or copolymer of tetrafluoroethylene and a monomer which is copolymerizable therewith, wherein the mole ratio of fluorine to hydrogen is at least 1:1, and which is solid at the melt-processing temperature of the difficultly melt-processable polymer.
Patent Literature 7 discloses a low-temperature-decomposable engineering plastic resin composition which is conducive to improved molding processability in consideration of extrusion pressure, extrusion torque and other process parameters in the molding of low-temperature-decomposable engineering plastics. The low-temperature-decomposable engineering plastic resin composition prepared by formulating a low-temperature-decomposable engineering plastic with a fluorine-containing resin, wherein said low-temperature-decomposable engineering plastic has a melting point of not higher than 200° C. and a decomposition temperature of not higher than 300° C., and wherein said fluorine-containing resin is a resin comprising a fluorine-containing polymer, said fluorine-containing polymer having a fluorine atom and at least one atom species selected from the group consisting of hydrogen atom, chlorine atom, bromine atom and iodine atom, said fluorine atom and said at least one atom species being bound to a non-terminal carbon atom constituting a main chain, and said fluorine-containing polymer having substantially no polar functional groups reactive to the low-temperature-decomposable engineering plastic.